ACHRNEWS

Technical Savvy and Three-Way Valves

June 12, 2000
More and more applications are using heat reclaim as a means of providing a supplementary or even a primary heat source. Heat reclaim can significantly lower energy costs.

Heat reclaim is best described as the process of reclaiming heat that would normally be rejected by an outdoor condenser. Typically, the refrigerant is diverted to an air handler in an area that requires heat.

One of the older applications of heat reclaim is in a supermarket, since a supermarket has a constant supply of heat that is removed from the many refrigerated display fixtures and coolers.

Today there are many cost-effective applications of heat reclaim in refrigeration, air conditioning, dehumidification, and heat pump systems.

Types of Reclaim

While the most popular application of heat reclaim is heating air, water heating is popular in supermarkets, convenience stores, and restaurants, which all use considerable amounts of hot water.

Essentially, any application that requires heat can recover the heat from a refrigeration or air conditioning system. The energy efficiency of recovered heat will almost always be more efficient than any other purchased heat source.

The common-sense question is, “Why reject heat to the outdoors when additional heat is required in any other moderate temperature application within the system or building?”

Three-way refrigerant heat reclaim valves make it convenient to recover rejected or waste heat.

Series vs. Parallel Piping Schematics

Figure 1 shows typical piping schematics for two basic types of piping arrangements, series and parallel condensers. The selection of the piping arrangement depends upon the sizing of the reclaim coil and the control scheme of the system.

If the parallel piping arrangement is used, the reclaim condenser must be sized to handle 100% of the rejected heat at the conditions and time at which the reclaim coil is being used.

If the series piping arrangement is used, care and safety measures should be taken to prevent the mixing of subcooled refrigerant with hot gas vapors. These safety measures could include pressure or temperature lockout controls and time delay relays.

Bleed Ports

Three-way heat reclaim valves with three-way pilot valves are available in a variety of sizes. These valves are typically available with an optional bleed port. (See Figure 2.)

The bleed port allows the refrigerant to be removed from the heat reclaim coil or heat exchanger when it is not being used.

There are two reasons why the refrigerant is removed from the heat reclaim coil. One is to maintain a proper balance of refrigerant in the system (i.e., refrigerant left in the reclaim coil could result in the remainder of the system operating short of charge).

A second reason is to eliminate the potential for having condensed refrigerant in an idle coil. When an idle reclaim coil has condensed, or even subcooled liquid refrigerant is sitting in the tubes, there is a potential for a problem.

When refrigerant liquid, either saturated or subcooled, is mixed with hot gas refrigerant, the reaction of the mixing can cause severe liquid hammer. Hot gas mixed with liquid can create thousands of pounds of force and has the potential of breaking refrigerant lines and valves.

An alternate method of removing the refrigerant from a heat reclaim coil is to use a separate normally open solenoid valve and an optional fixed metering device. (See Figure 3.)

The separate solenoid valve allows the flexibility of pumping out the reclaim heat exchanger refrigerant as a liquid instead of a vapor. There are two benefits to pumping out the reclaim coil as a liquid:

1. Removal of any oil that may be present in the reclaim heat exchanger; and

2. The refrigerating effect of the liquid can be used to lower the superheat of vapor entering the compressor, instead of cooling the heat reclaim heat exchanger.

Note: A check valve should be installed in the heat reclaim pump out or bleed line whenever the reclaim heat exchanger is exposed to temperatures lower than the saturated suction temperature of the system. This will prevent migration of refrigerant to the coldest location of the system.

Three-Way Split Condenser Valves

The split condenser valve is a relatively simple modification of the standard heat reclaim valve.

Figure 4 shows a split condenser valve. Its upper seat and port of the split condenser valve opens and closes. The lower port is always open. During the normal full condenser mode, the refrigerant flow is split evenly between the two halves of the condenser.

When employing heat reclaim in series with the outdoor condenser of a refrigeration system, the required refrigerant charge could be a potential problem. If most or all of the heat is removed from the refrigerant in the heat reclaim coil, some or all the refrigerant may be in the form of liquid when it enters the outdoor condenser coil. During this condition, the liquid charge in the system would have to be large enough to completely fill the condenser with liquid.

The requirement for this charge may be reduced by splitting and using only half of the outdoor condenser.

During winter, the effectiveness of the condenser surface area is much greater than it is during summer. Typically, split condenser valves are controlled by an ambient temperature control set for a specific outdoor temperature. This usually coincides with the requirement for heat reclaim in the building.

In some applications, the control system will also split the condenser anytime the heat reclaim coil is active.

Sizing for Split Condenser applications

A typical capacity table for three-way heat reclaim valve selection is shown in Figure 6. These capacities are based on net evaporating capacity in tons.

Heat reclaim and split condenser valves are pilot-operated three-way valves. Most three-way valves also have three-way pilot operators. The three-way pilot uses both suction and discharge pressure to shift the valve. Therefore, a valve will always shift regardless of whether it is oversized or undersized.

These valves will shift independently of the pressure drop through the main port of the valve. Minimum pressure drops through three-way valves save energy by allowing the system to operate at lower discharge pressures.

Heat reclaim valves are typically sized for a pressure drop that is equivalent to a 1°F (or less) change in saturation temperature. This corresponds to approximately 3 psi for R-22.

Split condenser valves, similar to heat reclaim valves, do not require a pressure drop to shift. Typically these valves are also sized for a change in pressure drop equivalent to a change in saturation temperature of 1° or less.

It is important to have equal flow to both halves of a split condenser. When a condenser and its associated piping are equally balanced, the split condenser valve will also provide balanced flow with little or no pressure drop.

If the piping of the split condenser cannot be equally balanced, the split condenser valve may be selected with a higher pressure drop to help balance the flow equally to both halves of the condenser. Jones is product manager, Electrical Products, Sporlan Valve Co., Washington, MO.

Sidebar: Use Head Pressure Controls

When employing heat reclaim on a refrigeration system, the addition of head pressure controls is important not only to maintain liquid pressure at the expansion valve inlet, but also to ensure the availability of quality hot gas at the reclaim heat exchanger.

Typically, the additional energy required to maintain a higher head pressure is not enough to pay for an alternate method of heating.